1. Field of the Invention
The present invention relates to an X-ray mask blank, an X-ray mask, and a pattern transfer method used for X-ray lithography.
2. Description of the Related Art
In the semiconductor industry, as a technique for transferring a fine pattern to form an integrated circuit composed of a fine pattern on a silicon substrate or the like, a photolithography method has been hitherto used in which the fine pattern is transferred using visible light or ultraviolet light.
In recent years, however, with the advances of the semiconductor technology, the integration scale of super-LSIs or other semiconductor devices is growing higher. This has led to a demand for a high-precision fine pattern transfer technique which breaks through the limitations of the transfer technique that depends on visible light or ultraviolet light conventionally used in the photolithography method.
To implement the transfer of such a fine pattern, an X-ray lithography method using X-rays shorter in wavelength than visible light or ultraviolet light is being developed.
The configuration of an X-ray mask employed for the X-ray lithography is shown in FIG. 1.
As shown in the drawing, an X-ray mask 1 is constituted by an X-ray transparent film or membrane 12, through which X-rays are transmitted, and an X-ray absorber pattern 13a for absorbing X-rays; these components are supported by a support substrate or frame 11a made of silicon.
FIG. 2 shows the configuration of an X-ray mask blank. An X-ray mask blank 2 is composed of the X-ray transparent film 12 and an X-ray absorber film 13 formed on a silicon substrate 11.
For the X-ray transparent film, silicon carbide having high Young's modulus and exhibiting high resistance to the exposure to X-rays is commonly used. For the X-ray absorber film, an amorphous material containing Ta which is highly resistant to the exposure of X-rays is frequently used.
The X-ray mask 1 is fabricated from the X-ray mask blank 2 by, for example, the following process.
A resist film on which a desired pattern has been formed is placed on the X-ray mask blank 2, then dry etching is performed using the resist pattern as the mask to form an X-ray absorber pattern. After that, the film of the area which corresponds to a window area (the recessed portion on the back surface) of an X-ray transparent film formed on the back surface is removed by a reactive ion etching (RIE) process which employs CF.sub.4 as the etching gas. The remaining film is used as the mask to etch the back surface of the silicon substrate by using an etchant composed of a mixture of hydrofluoric acid and nitric acid.
In the process mentioned above, an electron beam (EB) resist is usually used as the resist; the pattern is formed by exposure using an EB writing process.
The EB resist, however, does not have sufficiently high resistance to dry etching, which is quick etching, used for processing the X-ray absorber film. Hence, if the X-ray absorber film is directly etched using the resist pattern as the mask, then the resist pattern is lost by etching before the formation of the pattern on the X-ray absorber film is completed, making it impossible to obtain the desired X-ray absorber pattern.
As a general solution to the foregoing problem, a film known as an etching mask layer having a high etching selective ratio for the X-ray absorber film is inserted between the X-ray absorber film and the resist in order to form the X-ray absorber film pattern.
In such a case, to prevent a difference in size from being produced between the resist pattern and the X-ray absorber pattern, which difference is referred to as "pattern conversion difference," it is necessary to make the etching mask layer as thin as possible. For this reason, when patterning the X-ray absorber film, it is required to set the speed for etching the etching mask layer sufficiently low (a high etching selective ratio) in relation to the speed for etching the X-ray absorber film.
In addition, the X-ray absorber film must be etched for a slightly longer than a preset time, which is known as "over-etching" so as to ensure a uniform pattern configuration in a wafer surface without leaving partially unetched portion on the mask surface.
The over-etching causes the X-ray transparent film, which is the bottom layer of the X-ray absorber film, to be exposed to plasma. If the bottom layer of the X-ray absorber film is, for example, an X-ray transparent film composed of a silicon carbide, then the etching speed for the X-ray transparent film exceeds a negligible speed in relation to the etching conditions of the X-ray absorber film. Hence, the X-ray transparent film is over-etched, leading to a thinner bottom layer, namely, the X-ray transparent film, and a deteriorated pattern configuration of the X-ray absorber film itself. The thinner X-ray transparent film undesirably causes a change in the optical transmittance required for the alignment when mounting the film on an X-ray aligner, or adds to the positional distortion of the mask.
Therefore, it is preferable to insert an etching stopper layer between the X-ray absorber film and the X-ray transparent film, the etching stopper layer being made of a material which is hard to be etched (which has a high etching selective ratio) when etching the X-ray absorber film.
Hitherto, chlorine gas has been used for etching an X-ray absorber film containing Ta as a chief ingredient thereof, while a Cr film has been used as the etching mask layer and the etching stopper layer that enable a high etching selective ratio for the X-ray absorber film. A fluoride gas such as SF.sub.6 has been used for etching the X-ray absorber film which has W as the chief ingredient thereof, and the Cr films have been used for the etching mask layer and the etching stopper layer for the X-ray absorber film. These Cr films are formed on the bottom and/or the top of the X-ray absorber film by the sputtering method in most cases.
High positional accuracy is required of the X-ray mask; for instance, the distortion of the X-ray mask for a 1-Gbit DRAM which has a 0.18 .mu.m design rule pattern must be controlled to 22 nm or less.
The positional distortion is heavily dependent on the stress of the material of the X-ray mask; if the stress of the X-ray absorber film, the etching mask layer, or the etching stopper layer is high, then the positional distortion is provided. Hence, the stress of the X-ray absorber film, the etching mask layer, and the etching stopper layer must be minimized.
No satisfactory study, however, has been performed on the stress of the X-ray masks for the DRAMs of 1 Gbits or more.